This invention generally relates to stranded wires used in orthodontic, medical and other applications, and more specifically, to coil springs manufactured from stranded wire.
Stranding stainless steel wires is known for reducing the stiffness of the stainless steel wires. By stranding is meant that two or more strands of wire are twisted or braided together. The more strands per inch, the lower the stiffness of the stranded wire. Stranding wires is not known for purposes other than stiffness reduction. NiCr wires are likewise stranded to reduce their inherently high stiffness. For a superelastic or shape memory alloy, however, such as a Ni/Ti based alloy, the material inherently has a low stiffness, so the use of stranded wires would not be necessary for stiffness reduction. Ni/Ti wires are becoming increasingly popular for numerous applications, including dental and medical devices. For example, these low stiffness Ni/Ti wires are being used in orthodontic brackets and medical stents.
Coil springs also have many applications in medical and dental industries. For example, in the field of orthodontics, coil springs have replaced elastics xe2x80x9cOxe2x80x9d rings or soft stainless steel ligature ties in the ligation of arch wires and retainer wires in slots of orthodontic brackets. Also in the field of orthodontics, coil springs are being used in bite fixing appliances to push a patient""s lower jaw forward or backward to correct Class 2 and Class 3 malocclusions, i.e., overbites and underbites. Coil springs are also being used in the medical field for stents or devices used in laproscopic surgeries.
Frequently, the coil springs used in oral applications, such as the ligating members or bite fixing coils, will break in the mouth over time due to fatigue. When this happens, the clinicians must remove the appliance and replace it with a new unit, which is very time consuming and costly for the orthodontist. Furthermore, fracture of the coil spring may cause irritation to the patient, as well as prolong the length of treatment due to the appliance ceasing its function after the fracture occurs. Likewise, single strand wires, including Ni/Ti wire, are also subject to premature failure from fatigue. In addition to orthodontic applications, premature failure in single strand and coiled wires due to fatigue is a disadvantage in any application of these wires.
There is thus a need to develop wires having improved fatigue resistance in any environment in which the wires are used.
The present invention provides a stranded wire having improved fatigue resistance as compared to a single strand of wire. The stranded wire comprises at least two individual strands twisted or braided together, the individual strands being of a shape memory alloy and/or superelastic material. For example, the strands of wire may be made of Ni/Ti or an alloy thereof, preferably comprising at least 50% titanium by weight, and more preferably comprising at least 40% nickel by weight. The stranded wire of the present invention may be a fishing wire, a medical device, a dental device, or any other wire that benefits from improved fatigue resistance in use. In a further embodiment of the present invention, at least a portion of the stranded wire is coiled to form a coil spring. The stranded and coiled wire may be an orthodontic bite fixing device, a stent, a ligature in an orthodontic bracket, an archwire in an orthodontic bracket, or any other coil spring that benefits from improved fatigue resistance in use. The present invention further includes a method of making high fatigue resistant wires, comprising twisting together multiple strands of wire and heat treating the twisted wires to set the shape of the twisted configuration.